Non-aqueous electrolyte secondary batteries, particularly, lithium ion secondary batteries have a high operating voltage and a high energy density. Thus, development for lithium ion secondary battery has been spurred recently, as a power source for electrically-powered tools or vehicles which necessitate high output, in addition to a power source for driving portable electronic devices such as mobile phones, laptop computers, and camcorders. Particularly, high capacity lithium ion secondary batteries have been developed actively as a power source for replacing available nickel-metal hydride storage batteries to be used for hybrid electric vehicles (HEV).
The power source for HEVs needs to have a higher capacity, compared with the power source for small household devices. Battery modules including a plurality of substantially rectangular batteries stacked are preferably used for a power source for HEVs, since a high capacity can be obtained even with a small size module.
In such battery modules, batteries tend to be affected by a binding force for retaining the module dimension, especially the batteries positioned in the center area. Under such effects from the binding force, in non-aqueous electrolyte secondary batteries using micro-porous film comprising resin such as polyolefin as the separator, for example, when a pressure is applied to the battery, the non-aqueous electrolyte is easily forced out from the separator. In fact, when a substantially rectangular lithium ion secondary battery using a separator made of resin is molded with resin, the binding force by the molding resin forces the non-aqueous electrolyte out from the separator. As a result, ion conductivity is lost in the separator to decline the battery performance.
There has been proposed to use a highly stiff porous heat-resistant layer comprising a nonconductive filler such as silica and a binder such as polyvinylidene fluoride, instead of a conventional resin-made separator (Japanese Laid-Open Patent Publication No. Hei 10-106530).
In battery modules, several tens of substantially rectangular non-aqueous electrolyte secondary batteries are usually bound. The electrodes expand when the batteries are charged, the battery case tries to expand. However, in battery modules, each battery cannot freely change its shape since all the batteries are bound, and the force caused by the battery case deformation is likely to concentrate on the center of the module. Thus, in the battery positioned at around the center of the module, a load of 100 kgf/cm2 (average 20 kgf/cm2) at the maximum is applied at the ends of the battery in the thickness direction thereof while charging.
The inventors of the present invention found in their examination that under such harsh environment, the problem of the shortage of the non-aqueous electrolyte in the porous heat-resistant layer cannot be solved just by using the technique disclosed in Japanese Laid-Open Patent Publication No. Hei 10-106530.
The present invention was made in view of the above problems, and aims to provide a reliable non-aqueous electrolyte secondary battery which can avoid the forcing out of the non-aqueous electrolyte from the electrode assembly, even under the environment where the size change in the battery case is unacceptable.